Method and system for two-dimensional interferometric radiometry

ABSTRACT

A method and system are disclosed for imaging a planetary surface region of interest (ROI). In a primary application, a plurality of space vehicles having antennas mounted thereupon are utilized to collect thermal radiation emitted from the ROI and generate corresponding thermal emission signals. Such thermal emission signals may be combined to yield one or more simple interferometric fringes. The simple fringes may be employed to yield a pixel image of the ROI. In one aspect, one or more simple interferometric fringes may be utilized to generate one or more compound interferometric fringes for use in formation of the pixel image. One or more compound fringes may be utilized to generate additional levels of compound fringes for use in formation of the pixel image. In another aspect, the space vehicles may be positioned in a “near-field” imaging arrangement relative to the ROI and a matched filtering approach may be utilized for extracting amplitude data from the interferometric fringe(s) on a basis for use in pixel image formation.

RELATED APPLICATIONS

[0001] This application is a continuation application of U.S. patentapplication Ser. No. 09/731,113, filed Dec. 6, 2000, which claimspriority to U.S. Provisional Patent Application Serial No. 60/169,484,filed Dec. 7, 1999, and to U.S. Provisional Patent Application SerialNo. 60/219,157, filed Jul. 19, 2000, the entirety of which applicationsare hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to imaging systems in which two ormore complex signals of a region of interest may be combined to yieldone or more interferometric images. More particularly, the invention isdirected to a method and system for two-dimensional radiometric imagingof a planetary surface region of interest utilizing thermal radiationemitted by the region of interest.

BACKGROUND OF THE INVENTION

[0003] Computed imaging systems are utilized in a wide variety ofapplications. Of particular interest here is the use of radio frequencyantennas to collect complex signals employable to obtain high qualityimages of planetary surfaces.

[0004] Such complex images are typically obtained by overheadtransmission/reflected receipt of pulses of energy at a predeterminedfrequency. In the latter regard, microwave radiation has beenadvantageously employed due to its ability to yield high resolutionimages in virtually all weather conditions and at all times (i.e., dayand night).

[0005] While such systems have been utilized with success, they requirethe use of radiation signal transmission payloads on one or moreaircraft or satellites (i.e., “space vehicles”). As may be appreciated,such transmission payloads add significant weight, complexity and costto an imaging system. Additionally, the use of active transmittersentails significant attendant power requirements. Further, the activetransmission of microwave signals toward a region of interest isdetectable and may be undesired in certain applications.

SUMMARY OF THE INVENTION

[0006] In view of the foregoing, a primary objective of the presentinvention is to provide an improved imaging system and method thatreduces imaging componentry payload and complexity on space vehiclesutilized to collect imaging data. Related objectives are to reduceon-board power requirements and componentry costs associated with theobtainment of imaging data on space-borne vehicles.

[0007] Another important objective of the present invention is toprovide a radiometric imaging system and method that is passive innature and thereby avoids the active transmission of energy signals toan image region of interest to form a pixel image thereof.

[0008] An additional main objective of the present invention is toprovide an imaging system and method that reduces the number of spacevehicles and associated antennas necessary for generatinghigh-resolution images.

[0009] Yet another objective of the present invention is to provide animaging system and method that provides high-resolution images ininclement weather and day/night conditions.

[0010] The above objectives and additional advantages are realized bythe present invention. To do so, the present inventors have recognizedthat even though thermal emissions from a planetary surface region ofinterest are of random phase and amplitude, such emissions may beassumed to be largely isotropic and mutually coherent at a receivingantenna (e.g., as received or time-shifted), and may be collected andprocessed in a manner that allows such randomness to be effectivelyremoved. Relatedly, it has been recognized that thermal radiationcollection and processing can be carried out in a manner that reducesthe number of antennas necessary to yield high-resolution images. At theoutset it should be noted that while the present invention isparticularly apt for radiometric imaging applications, certain aspectsmay also be employable in active imaging arrangements.

[0011] The inventive system contemplates a plurality of space vehicleslocated in known relative positions over a planetary surface region ofinterest (ROI). At least a corresponding plurality of antennas aremounted on the space vehicles to collect radiation emissions from theROI (e.g., thermal or blackbody radiation) and provide correspondingthermal emission signals. In turn, processor means (e.g., one or moresignal processors) may be utilized (e.g., either on-board the spacevehicles and/or more preferably at another location) to combine thethermal emission signals and obtain interferometric fringe signalsemployable to form a pixel image of the ROI. As will be appreciated, theformation and use of interferometric fringes effectively removes phaserandomness from the collected signals.

[0012] In one aspect of the invention, the space vehicles may be spacedat different relative distances therebetween, wherein the collectionantennas collectively define a “sparse aperture”. For such purposes, thespace vehicles may be located so that two or more of the antennas arehorizontally and/or vertically offset from each other in relation to theimaged ROI during imaging. Such an arrangement allows the thermalemission signals obtained by the antennas to be processed in varyingcombinations, wherein each combination yields a differentinterferometric phase measurement based upon a corresponding differentinterferometric baseline. As such, the multiple differentinterferometric phase measurements can effectively “fill-in” an array ofinterferometric images employable in pixel image formation for the ROI.As will be appreciated, the differential spacing of antennas tocollectively define a sparse aperture facilitates reduction of theoverall number of space vehicles required to yield high-resolution ROIimages.

[0013] In a further aspect of the invention, the space vehicles may bepositioned in a “nearfield” imaging arrangement to collect thermalemissions from an ROI. That is, the space vehicles may be positioned sothat the imaging center axes for at least two of the antennas define anangle θ of at least about 2° therebetween, and more preferably about 2°and 15° therebetween, depending upon the collection center frequency ofthe antennas. In the latter regard, the antennas may be provided tocollect thermal emissions over a collection bandwidth of between about 1MHz and 1 GHz with a center frequency of between about 1 GHz and 100GHz. The establishment of a near-field imaging arrangement alsofacilitates the obtainment of high-resolution ROI images.

[0014] In one arrangement, a plurality of antennas may be mounted on acorresponding plurality of satellites located in a known constellationpassing over an ROI to be imaged. More particularly, two or moresatellites may be located in corresponding repeatable orbits havingrelatively small differences in eccentricity and/or inclination (e.g.,Hill's orbits), wherein the corresponding antennas are horizontallyand/or vertically offset in a known geometry relative to the ROI forimaging. By way of example, four satellites may be positioned in knownorbits to laterally define a repeatable Y-shaped pattern for sparseaperture imaging. Further, at least two of the satellites may bepositioned so that the center imaging axes of the corresponding antennasmounted thereupon define an angle of about 2° to 15° therebetween,thereby yielding a near-field imaging arrangement. In earth imagingapplications, the satellites may be disposed in low-earth orbits,wherein the satellites are placed at altitudes and spacings consistentwith near-field operations.

[0015] In conjunction with noted aspects of the present invention, itshould be recognized that the antennas should be provided in a spotlightmode (e.g., via gimbaled mounting) so that they remain pointed at animaged ROI during an imaging, or “dwell”, time period. Further in thisregard, the antennas should be provided to collect thermal emissionsfrom overlapping portions of the ROI in a substantially simultaneousmanner to maintain mutual coherence. In turn, the thermal emissionscollected from the ROI at each of the antennas may be substantiallysimultaneously sampled at a predetermined frequency (e.g., at least theNyquist rate) during a given dwell period, thereby yielding an ROIthermal emission data set comprising each thermal emission signal. SuchROI data sets are combinatively employed by the processor means forimage formation.

[0016] In the latter regard, and in another general aspect of thepresent invention, the processor means may be provided to combine, orcorrelate, at least a first thermal emission signal (e.g., collected bythe first antenna) with a complex conjugate of at least a second thermalemission signal (e.g., collected by a second antenna) to obtain at leasta first “simple” interferometric fringe signal. As will be appreciated,it is generally preferable therefore to correlate a plurality ofdifferent pairs of thermal emission signals to obtain a plurality ofdifferent simple interferometric fringe signals. Further, and in orderto enhance mutual coherence, it may be desirable in certain applicationsto time-shift one of the thermal emission signals of a given pair priorto correlation (e.g., in high bandwidth applications).

[0017] After formation, each simple interferometric fringe signal(s) maybe low-pass filtered to yield a corresponding averaged, or “smoothed”,signal, wherein amplitude randomness is effectively removed. In turn,the processor means may correlate at least the first simpleinterferometric fringe signal and at least one other signal for the ROI(e.g., obtained/generated pursuant to the corresponding-in-time receiptof thermal emissions from the ROI) to obtain at least one “compound”interferometric fringe signal employable in the formation of a pixelimage of the ROI. The signal that is combined with the first simpleinterferometric fringe signal may be one of a first thermal emissionsignal, second thermal emission signal or third thermal emission signal(e.g., collected by a third antenna), or perhaps more preferably, asecond simple interferometric fringe signal obtained by combining one ofthe first and second thermal emission signals with a complex conjugateof a third thermal emission signal. As will be appreciated, it isgenerally preferable to form a plurality of compound interferometricfringe signals for use in image formation.

[0018] By way of example, a first simple interferometric fringe signal(e.g., which correlates thermal emission signals generated by first andsecond antennas) may be combined with a second simple interferometricfringe signal (e.g., which correlates thermal emission signals generatedby third and fourth antennas) to obtain a first compound interferometricfringe signal. Similarly, simple fringe correlations of the first andthird thermal emission signals and of the second and fourth thermalemission signals can be further correlated to obtain a second compoundinterferometric fringe signal. Further, the first and second compoundinterferometric fringe signals may be further combined in an additionalstage. The formation/utilization of simple interferometric fringes andcompound interferometric fringes particularly facilitates a sparseaperture imaging arrangement, thereby reducing the number of spacevehicles/antennas needed to generate high-resolution images.

[0019] In yet a further aspect of the present invention, the processormeans may be provided to extract pixel values therefore, (e.g., complexvalues (i.e., comprising phase and amplitude components) or realamplitude values) from at least one and preferably a plurality ofinterferometric fringe signals employed for image formation on a perpixel location basis. In turn, the extracted pixel values are employableby the processor means to “develop” the pixel image of the ROI. By wayof primary example, for each different interferometric fringe signalemployed for ROI pixel image formation, the processor means may providefor the corresponding application of a plurality of different matchedfilters (e.g., corresponding with each of a plurality of pixel locationsfor the ROI pixel image to be formed) to obtain a plurality of extractedpixel values in corresponding relation to each of the plurality of pixellocations. In turn, the pixel values corresponding with each given pixellocation may be utilized to form an interferometric image signal (e.g.,for each interferometric fringe signal employed for image formation). Inone arrangement, for each given one of a plurality of interferometricfringe signals employed, the extracted pixel values for each given pixellocation may be combined to obtain a corresponding interferometric imagesignal. The plurality of interferometric image signals correspondingwith the plurality of interferometric fringe signals employed may bemerged (e.g., via complex summation and/or simple or weighted averaging)to yield the ROI pixel image. The utilization of separate matchedfilters for each pixel location and each interferometric fringe signalemployed facilitates near-field imaging of an ROI as discussed above.

[0020] In view of the foregoing, it will be appreciated that aninventive method may comprise the steps of collecting thermal emissionfrom a planetary surface region of interest (ROI) by a plurality ofspaced antennas to obtain a corresponding plurality of thermal emissionsignals. Following collection, the method further includes the step offirst combining at least a first thermal emission signal with a complexconjugate of at least a second thermal emission signal to obtain atleast a first simple interferometric fringe signal. Preferably, aplurality of different simple interferometric fringe signals are formedfrom different pairs of collected thermal emission signals, wherein thedata comprising one of each such pairs may be time-shifted to maintainmutual coherence. Each simple interferometric fringe signal may below-pass filtered to remove undesired high-frequency components andotherwise yield an averaged signal.

[0021] In one aspect, the inventive method may further comprise the stepof second combining at least a first simple interferometric signal withanother signal for the ROI (e.g., a signal generated fromcorresponding-in-time thermal emissions from the ROI) to obtain at leasta first compound interferometric fringe signal. In this regard, thesecond combining step may provide for the combining of one of the firstand second thermal emission signals with the complex conjugate of athird thermal emission signal to obtain a second simple interferometricfringe signal. As such, the noted first compound interferometric fringesignal may be generated by combining a first simple interferometricfringe signal with one of (i) the second simple interferometric fringesignal, and (ii) one of said first, second and third thermal emissionimage signals. Preferably, a plurality of different compoundinterferometric fringe signals are formed. As noted, the formation/useof one or more compound interferometric fringe signals in imageformation facilitates the use of a sparse aperture arrangement.

[0022] In another aspect the inventive method may provide for (i)collecting thermal radiation at the collection antennas over apredetermined frequency bandwidth of about 1 MHz to 1 GHz, and (ii)positioning at least two of the collection antennas to define an angleof at least about 2° between their respective center imaging axes (e.g.,to define a near-field imaging arrangement). Further, for at least afirst interferometric image signal, and more preferably for each givenone of a plurality of interferometric fringe signals (e.g., simpleand/or compound), the inventive method may include the step of applyinga different matched filter corresponding with each of a plurality ofpixel locations to interferometric signal data to extract a plurality ofpixel values (e.g., complex values or real amplitude values)corresponding with each of said plurality of image pixel locations.Then, for each interferometric fringe signal employed, the inventivemethod may include the step of combining the pixel values correspondingwith each of the plurality of pixel locations (e.g., summing) andutilizing the combined pixel values to obtain an interferometric imagesignal employable in the formation of the pixel image of the ROI. Wherematched filtering is applied to a plurality of different interferometricfringe signals, the resultant plurality of interferometric imagessignals may be merged to yield the ROI pixel image. By way of example,such merging may provide for the averaging or weighted averaging of thedifferent interferometric images in generating the ROI pixel image.

[0023] Additional aspects and advantages of the present invention willbe readily apparent to those skilled in the art after consideration ofthe further description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 illustrates one system embodiment of the present invention.

[0025]FIG. 2 illustrates satellite ranges for the system embodiment ofFIG. 1.

[0026]FIG. 3 is a schematic illustration of a processor means embodimentemployable in the system of FIG. 1.

[0027]FIG. 4 is a process diagram showing steps employable in the systemembodiment of FIG. 1.

DETAILED DESCRIPTION

[0028]FIG. 1 illustrates an exemplary system embodiment 10 of thepresent invention. System 10 includes four space vehicles 20 a, 20 b, 20c and 20 d, each having a collection antenna 22 a, 22 b, 22 c and 22 dmounted thereupon for receipt of thermal radiation emitted from a regionof interest (ROI) on a planetary surface (e.g., within a circular accessregion). In the latter regard, antennas 22 a, 22 b, 22 c and 22 d shouldbe provided for operation in a spotlight mode, e.g., wherein theantennas are gimbaled relative to the space vehicles to maintain asubstantially common footprint over the ROI during imaging operations.The exemplary system 10 further includes a communications space vehicle30 and ground station 32 for the transmission of control informationand/or downloading of thermal emission data collected by antennas 22 a,22 b, 22 c and 22 d.

[0029] In the latter regard, exemplary system 10 may be provided so thatthermal emission data collected at space vehicles 20 b, 20 c and 20 dmay be relayed to space vehicle 20 a for downloading via acommunications antenna 24 a to ground station 32 and subsequentprocessing for image formation. In alternate arrangements, the data maybe uploaded to and at least partially processed at communications spacevehicle 30, then downloaded to ground station 32 for further processing.In yet another arrangement, each of the space vehicles 20 a, 20 b, 20 cand 20 d may directly transmit the corresponding collected data forprocessing at the communications space vehicle 30 and/or ground station32.

[0030] Space vehicles 20 a, 20 b, 20 c and 20 d may be located in knownrelative positions for imaging. By way of primary example, spacevehicles 20 a, 20 b, 20 c and 20 d may comprise four satellites in aknown constellation. That is, the satellites may be disposed inclosely-related and repeatable orbits, wherein at least two of thesatellites are horizontally offset and at least two of the satellitesare vertically offset in a known geometry during imaging. Suchhorizontal and vertical spacing is preferred so that a predeterminedresolution can be achieved with antennas 22 a, 22 b, 22 c and 22 dcollectively defining a sparse aperture arrangement. More particularly,such arrangement allows four sets of thermal emission data collected byantennas 20 a, 20 b, 20 c and 20 d to be processed in variouscombinations (e.g., six different simple combinations of two), whereineach combination yields a different interferometric phase measurementbased upon a corresponding different baseline.

[0031] For example, FIG. 2 illustrates satellites 20 a, 20 b, 20 c and20 d in an arrangement in which antennas 22 a, 22 b, 22 c, and 22 d aredisposed at corresponding ranges R1, R2, R3 and R4 relative to an ROIcenter point along their respective center imaging axes. By way ofexample, range differences between R1 and R2, R2 and R3, and R3 and R4yield corresponding different baselines ΔR_(1,2), ΔR_(2,3) and ΔR_(3,4)for purposes of interferometric measurements. Similarly, rangedifferences between RI and R3, RI and R4, and R2 and R4 yield additionaldifferent baselines for interferometric measurements.

[0032] In addition to the noted vertical/horizontal spacing, spacevehicles 20 a, 20 b, 20 c and 20 d may also be positioned to define anear-field imaging arrangement. That is, and referring again to FIG. 2,space vehicles 20 a, 20 b, 20 c and 20 d may be positioned so that theimaging center axes for at least two of the antennas 22 a, 22 b, 22 cand/or 22 d define an angle θ of at least about 2° therebetween,preferably 2° and 15° therebetween, and most preferably between about 6°to 10° therebetween. Relatedly, the antennas 22 a, 22 b, 22 c and 22 dmay be provided to collect radiation over a bandwidth of about 1 MHz to1 GHz with a center frequency of about 1 GHz to 100 GHz. Further, inearth-imaging applications, space vehicles 20 a, 20 b, 20 c and 20 d maybe preferably positioned at elevations of no more than about 600 nmi,and most preferably between about 200 nmi and 500 nmi relative to a ROI.

[0033] In one configuration, four satellites may be located in low-earthorbits of about 400 nmi. To do so, the satellites may be carried by asingle launch vehicle to a predetermined orbital altitude, then deployedfrom the launch vehicle. In turn, thrusters on the satellites may beutilized as needed to define a known constellation with use of anon-board positioning system (e.g., a system referenced to the GlobalPositioning System (GPS)). By way of example, the satellites may bepositioned to laterally define a Y-shaped constellation relative to theearth surface. In such an arrangement, the satellites may be located inorbits that yield satellite spacings of between about 60 nmi and 120nmi.

[0034] During imaging, antennas 22 a, 22 b, 22 c and 22 d may becontrolled to substantially simultaneously collect thermal radiationemitted from the ROI over a common, predetermined dwell period. Further,and as previously noted, antennas 22 a, 22 b, 22 c and 22 d may beprovided to collect radiation in a spotlight mode over a collectionbandwidth of between about 1 MHz and 1 GHz, with a center frequency ofbetween about 1 GHz and 100 GHz. In turn, the collected radiation may besampled/digitized at antennas 22 a, 22 b, 22 c and 22 d (e.g., at a rateof between about 1.2 MHz and 1.2 GHz) for on-board thermal emission datastorage on space vehicles 20 a, 20 b, 20 c and 20 d, and subsequentdownloading/processing for image formation purposes.

[0035] In the latter regard, reference is now made to FIG. 3 whichschematically illustrates a processing means embodiment 100 employablein system 10. In the illustrated embodiment 100, the digitized thermalemission signals collected by antennas 22 a, 22 b, 22 c and 22 d may beintroduced for signal processing via corresponding channels 110 a, 110b, 110 c and 110 d. More particularly, the collected thermal emissionsignals 112 a, 112 b, 112 c and 112 d may be characterized as a functionof time (t) as follows:

S ₁(t)=a(t)ej(ω(t−R1/c)+φ(t))

S ₂(t)=a(t)ej(ω(t−R2/c)+φ(t))

S ₃(t)=a(t)ej(ω(t−R3/c)+φ(t)), and

S ₄(t)=a(t)ej(ω(t−R4/c)+φ(t)), respectively;

[0036] wherein a(t) defines the random amplitude and φ(t) defines therandom phase of thermal emissions from a ROI, and wherein ω is the radiofrequency within the operating band of the antennas 22 a-22 d and c isthe speed of light.

[0037] As illustrated in FIG. 3, the thermal emission signals 112 a, 112b, 112 c and 112 d in channels 110 a, 110 b, 110 c and 110 d may becombined in differing permutations, or sets, of two at combiners 120a-120 f to obtain interferometric phase differences therebetween. Thatis, combiners 120 a-120 f may provide for the mixing of correspondingdata samples comprising two different thermal emission signalsintroduced via channels 120 a, 120 b, 120 c, and 120 d to obtaincombined, or simple interferometric fringe signals 122 a-122 f. Whilenot shown, the data comprising one signal of any given pair of signals112 a, 112 b, 112 c and 112 d to be combined may be time-shifted inrelation to the other signal to ensure mutual coherence. In onearrangement signal mixing at combiners 120 a-120 f may be achieved viamultiplication of one thermal emission signal by the complex conjugateof another thermal emission signal. By way of example, when the signalson channels 110 a and 110 b are multiplied and the signals on channels110 c and 110 d are multiplied, the resultant simple interferometricfringe signals 122 a and 122 f, respectively, may be characterized asfollows:

S _(1,2)(t)=S ₁(t)·S ₂(t)*=a ²(t)e ^(jω(R2−R1)/c); and

S _(3,4)(t)=S ₃(t)·S ₄(t)*=a ²(t)e ^(jω(R3−R4)/c);

[0038] wherein “*” denotes the complex conjugate.

[0039] As may be appreciated, each combined signal 122 a-122 f maydefine a time-varying interference pattern, or simple fringe phasefunction, over an ROI. For example, the simple fringe phase functionscorresponding with the signals S_(1,2)(t) and S_(3,4)(t) noted above maybe characterized as follows:

φ_(1,2)(t)=2π(R2(t)−R1(t))/λ; and

φ_(3,4)(t)=2π(R4(t)−R3(t))/λ.

[0040] In the noted arrangement, multiplication of the various thermalemission signals 112 a, 112 b, 112 c and 112 d at combiners 120 a-120 fwill result in simple interferometric fringe signals 122 a-122 f, eachhaving a doubled bandwidth relative to that of the thermal emissionsignals introduced to the combiners 120 a-120 f. As such, in order tomaintain Nyquist integrity, and depending upon the radiation frequencycollected and sampling rate at antennas 22 a, 22 b, 22 c and 22 d,processor means embodiment 100 may further provide for the “upsampling”of the thermal emission signals 112 a-112 d (not shown) prior to signalcombining at combiners 120 a-120 f. Such up-sampling may be implementedvia software functionality that provides for the interpolation ofsuccessive data values comprising signals 112 a-112 d to increase thenumber of data values for further processing. After signal combining atcombiners 120 a-120 f, the resultant simple interferometric fringesignals 122 a-122 f may be low-pass filtered at filters 130 a-130 f(e.g., at a frequency of about 1 MHz or less) so as to remove highfrequency components and otherwise yield signal averaging. In the latterregard, such averaging serves to effectively remove amplitude randomnessfrom the collected thermal emissions.

[0041] As further illustrated by FIG. 3, the averaged simpleinterferometric fringe signals 132 a-132 f may be farther combined indiffering permutations, or sets, of two at combiners 140 a-140 o toobtain compound interferometric fringe signals 142 a-142 o. Inparticular, combiners 140 a-140 o may provide for the mixing ofcorresponding data values comprising two different averaged simpleinterferometric fringe signals 132 a-132 f (e.g., via multiplication),wherein each combination will further define a time-varying interferencepattern or compound fringe phase function, over an ROI. By way ofexample, the compound fringe phase function corresponding withmultiplication of the simple interferometric fringe signals S_(1,2)(t)and S_(3,4)(t) noted above may be characterized as follows:

φ_(1,2,3,4)(t)=φ_(1,2)(t)φ_(3,4)(t)=2π(R1(t)−R2(t)−R4(t)+R3(t))λ.

[0042] As may be appreciated, when combiners 140 a-140 o provide formultiplication of averaged simple fringe signals 132 a-132 f there willbe a resultant doubling of bandwidth. As such, the down-samplingfrequency at filters 130 a-130 f may be set to preserve Nyquistintegrity at combiners 140 a-140 f. Although not shown in FIG. 3, itshould also be noted that the formation of compound fringes at mixers140 a-140 o may alternately and/or additionally be obtained by themixing of any one of the thermal emission image signals 112 a, 112 b,112 c or 112 d with any one of the simple interferometric fringe signals122 a, 122 b, 122 c, 122 d, 122 e and/or 122 f.

[0043] Following formation of the compound interferometric fringesignals 142 a-142 o, the illustrated processing means embodiment 100provides for the application of matched filters 150 a-150 o to yieldcompound signals 152 a-152 o. More particularly, a separate matchedfilter corresponding with each of a plurality of pixel locations (e.g.,corresponding with the ROI pixel image to be formed) may be applied toeach of the data values comprising each of the compound interferometricfringe signals 142 a-142 o to extract pixel values (e.g.,amplitude-containing values) therefrom (e.g., via a simple dot productoperation).

[0044] In this regard, matched filters 150 a-150 o may be developed on aper pixel location basis in relation to the imaged ROI and theparticular combination of averaged simple interferometric fringe signals132 a-132 o and/or thermal emission signals 112 a-112 d combined atcombiners 140 a-140 b. Each such matched filter may effectively definethe predicted interferometic phase difference, or interferometric phasefunction, for the given pixel location and the given combination ofthermal emission signals 112 a-112 d and/or averaged simpleinterferometric fringe signals 122 a-122 o mixed for interferometricimage formation. As will be appreciated, such matched filters may bebased on known geometries between each of the antennas 122 a, 122 b, 122c and 122 d, in relation to the ROI, together with terrain data (e.g.,elevation data) for the ROI. Such terrain data may be predeterminedand/or otherwise derived on a dynamic basis from the complex imagesignals 112 a, 112 b, 112 c and 112 d (e.g., via a splitaperture/auto-focus technique).

[0045] Pursuant to or in conjunction with the application of matchedfilters 150 a-150 o, the resultant compound signals 152 a-152 o may befurther processed by integration modules 160 a-160 o to obtainintegrated interferometric image signals 162 a-162 o. In this regard,modules 160 a-160 o provide for integration (e.g., summation) of theextracted pixel values corresponding with each given pixel locationcorresponding with the ROI pixel image to be formed. In turn, theintegrated pixel values comprising each of the integratedinterferometric image signals 162 a-162 o combinatively define acorresponding interferometric image for use in ROI pixel imageformation.

[0046] That is, each of the integrated interferometric image signals 162a-162 o may be merged at merge module 170 to yield a compositeinterferometic image, or thermal image, of the ROI. In this regard, theintegrated interferometric image signals 162 a-162 o may be simplyaveraged at module 170. Alternatively, module 170 may provide forweighted averaging of the signals 162 a-162 o, wherein the signals 162a-162 o are weighted in a predetermined manner. A third alternative atmodule 170 may provide for complex summation of data comprising signals162 a-162 o.

[0047] Operation of the system embodiment 10 described above will now bebriefly summarized with reference to FIG. 4. Initially, thermalemissions from an ROI may be collected and sampled at a plurality ofantennas to obtain a corresponding plurality of thermal emission signals(Step 200). In relation to the above-described embodiment, antennas 22a-22 d located on corresponding satellites 20 a-20 d may be employed ina spotlight mode. As previously indicated, the satellites 20 a-20 d maybe located so that antennas 22 a-22 d are disposed in a near fieldimaging arrangement and so that the antennas 22 a-22 d collectivelydefine a sparse aperture.

[0048] Following obtainment of a plurality of thermal emission signals,at least one and preferably a plurality of simple interferometric fringesignals may be formed (Step 202). In relation to the above-describedembodiment, thermal emission signals 112 a-112 d may be multiplied atcombiners 120 a-120 f to form simple interferometric fringe signals 122a-122 f.

[0049] As illustrated in FIG. 4, the simple interferometric fringesignal(s) may be utilized to form a pixel image of the region ofinterest (Step 204). In particular, the simple interferometric fringesignal(s) may be filtered (Step 206) in order to remove high-frequencycomponents and correspondingly average each of the signal. In relationto the above-described embodiment, low-pass filters 130 a-130 f may beutilized for such signal averaging.

[0050] Then, at least two averaged simple interferometric fringe signalsmay be combined to obtain at least one compound interferometric fringesignal (Step 208). In relation to the above-described processingembodiment, the formation of compound interferometric fringe signal(s)142 a-142 o may entail the multiplication of averaged simpleinterferometric fringe signals 132 a-132 f at combiners 140 a-140 o. Aswill be appreciated multiple successive levels of compound fringesignals may be formed from compound fringe signals formed in a precedingstage.

[0051] Following the formation of the compound interferometric fringesignal(s), pixel values may be extracted therefrom for each of aplurality of pixel locations (Step 210). Such pixel locations correspondwith the ROI pixel image to be formed. As will be appreciated, theextracted pixel values may be utilized to generate a pixel image (Step212). In this regard, the extracted pixel values for each giveninterferometric fringe signal may be combined to generate acorresponding interferometric image or image signal. In turn, suchinterferometric image(s) may be employed to generate an ROI pixel image.

[0052] As noted in FIG. 4, the extraction of pixel values from theinterferometric fringe signals may entail the development of matchedfilters for each of a plurality of pixel locations in correspondingrelation to each given interferometric fringe signal employed for imageformation (Step 214). In turn, such matched filters may be applied incorresponding relation to each interferometric fringe signal on a pixellocation-specific basis (Step 216). In relation to the above-describedembodiment, matched filters 150 a-150 o may be employed for pixel valueextraction. Then, for each resultant signal 152 a-152 o the extractedpixel values for each given pixel location may be integrated atintegration modules 160 a-160 d to yield corresponding interferometricimage signals 162 a-162 o. In turn, signals 162 a-162 o may be merged atmodule 170 for pixel image formation.

[0053] The embodiment descriptions provided above are for exemplarypurposes only and are not intended to limit the scope of the presentinvention. Various adaptations, modifications and extensions of thedescribed arrangements will be apparent to those skilled in the art andare intended to be within the scope of the invention as defined by theclaims which follow.

What is claimed is:
 1. A system for obtaining an image of a planetarysurface region of interest, comprising: a plurality of space vehicleslocated in known relative positions over a planetary surface region ofinterest (ROI); a plurality of antennas, each located on a different oneof said plurality of space vehicles, for collecting thermal radiationemissions from said ROI and providing a corresponding plurality ofthermal emission signals in response thereto; and, processor means forcombining a first thermal emission signal with a complex conjugate of asecond thermal emission signal to obtain a first simple interferometricfringe signal and for using said first simple interferometric fringesignal and at least one other signal for said ROI to obtain a compoundinterferometric fringe signal employable in the formation of a pixelimage of said ROI.
 2. A system as recited in claim 1, wherein saidprocessor means is operable to combine one of said first and secondthermal emission signals with a complex conjugate of a third thermalemission signal to obtain a second simple interferometric fringe signal,and wherein said at least one other thermal emission signal is one of(i) said second simple interferometric fringe signal, and (ii) one ofsaid first, second and third thermal emission signals.
 3. A system asrecited in claim 1, wherein said plurality of space vehicles are definedby a corresponding plurality of satellites located in a knownconstellation.
 4. A system as recited claim 3, wherein at least two ofsaid plurality of satellites are horizontally offset and at least two ofsaid plurality of satellites are vertically offset for ROI imaging.
 5. Asystem as recited in claim 1, wherein center imaging axes for at leasttwo of said plurality of antennas define an angle of at least about 2°therebetween, and wherein said plurality of antennas collect saidthermal radiation emissions over a bandwidth of about 1 MHz to 1 GHz. 6.A system as recited in claim 5, wherein said plurality of space vehiclesare defined by a corresponding plurality of satellites located in earthorbits.
 7. A system as recited in claim 5, wherein said plurality ofsatellites are positioned so that said plurality of antennas are locatedat different distances from said ROI as measured along their respectivecenter imaging axes.
 8. A system as recited in claim 1, wherein saidprocessor means is operable to extract pixel values from said compoundinterferometric fringe signal, and wherein said extracted pixel valuesare employable by said processor means to form said pixel image of saidROI.
 9. A system as recited in claim 8, wherein said processor means isoperable to apply a different matched filter in corresponding relationto each of a plurality of pixel locations for said compoundinterferometric fringe signal.
 10. A system as recited in claim 1,wherein at least a portion of said processor means is remotely locatedrelative to said plurality of space vehicles.
 11. A system for obtainingan image of a planetary surface region of interest comprising: aplurality of space vehicles located in known relative positions over aplanetary surface region of interest (ROI); A plurality of antennas,each located on a different one of said plurality of space vehicles, forcollecting thermal radiation emissions from said ROI and providing acorresponding plurality of thermal emission signals in response thereto;and, processor means for utilizing said plurality of thermal emissionsignals to obtain a plurality of interferometric fringe signals,including at least one compound interferometric fringe signal, and foremploying said plurality of interferometric fringe signals to form apixel image of the ROI.
 12. A system as recited in claim 11, whereinsaid plurality of space vehicles are located so that center imaginingaxes for at least two of said plurality of antennas define an angle ofat least about 2° therebetween.
 13. A system as recited in claim 12,wherein said plurality of antennas collect said thermal radiationemissions over a bandwidth of about 1 MHz to 1 GHz with a centerfrequency of between about 1 GHz and 100 GHz.
 14. A system as recited inclaim 13, wherein said center imaging axes define an angle of betweenabout 2° and 15° therebetween.
 15. A system as recited in claim 14,wherein said plurality of space vehicles are positioned so that saidplurality of antennas are located at different distances from said ROIas measured along their respective center imaging axes.
 16. A system asrecited in claim 14, wherein at least two of said plurality of spacevehicles are horizontally offset and at least two of said plurality ofspace vehicles are vertically offset for ROI imaging.
 17. A system asrecited in claim 14, wherein said plurality of space vehicles includesfour space vehicles positioned to laterally define a Y-shapedconfiguration.
 18. A system as recited in claim 14, wherein said ROI islocated on earth, and said plurality of said space vehicles arepositioned at elevations less than about 600 nmi.
 19. A system asrecited in claim 18, wherein said space vehicles are located atelevations between about 200 nmi and 500 nmi.
 20. A system as recited inclaim 11, wherein said processor means is operable to combine aplurality of different pairs of said plurality of thermal emissionsignals to obtain a corresponding plurality of simple interferometricfringe signals.
 21. A system as recited in claim 20, wherein saidprocessor means is further operable to combine at least one of (i) apair of said plurality of simple interferometric fringe signals, and(ii) at least of one of said simple interferometric fringe signals andone of said thermal emission signals, to obtain said at least onecompound interferometric fringe signal.
 22. A system as recited in claim20, wherein said processor means is further operable to low-pass filtereach of said plurality of simple interferometric fringe signals toseparately average each of said simple interferometric fringe signals.23. A system as recited in claim 22, wherein said processor means isfurther operable to extract at least one pixel value in relation to eachof a plurality of pixel locations from each of least two of saidplurality of interferometric fringe signals to correspondingly obtain atleast two interferometric image signals.
 24. A system as recited inclaim 23, wherein said processor means is further operable to merge saidat least two interferometric image signals to obtain said pixel image ofthe ROI.
 25. A system as recited in claim 11, further comprising: acommunication space vehicle for receiving data corresponding with saidplurality of thermal emission signals from at least one of saidplurality of space vehicles and for downloading corresponding data to aremote location.
 26. A system as recited in claim 25, wherein saidprocessor means comprises: a first signal processor located on at leastone of said communication space vehicle and one of said plurality ofspace vehicles.
 27. A system as recited in claim 26, further comprising:a second signal processor located at said remote location.
 28. A systemas recited in claim 25, further comprising: communication antenna meanslocated on one of said plurality of space vehicles for receiving atleast some of said plurality of thermal emission signals from other onesof said plurality of space vehicles, and for transmitting said datacorresponding with said plurality of thermal emission signals to saidcommunications space vehicle.